AVONEX, a recombinant human interferon-beta product, is an antiviral protein produced by fibroblasts after induction by viral infection. This randomized, double-blinded, placebo-controlled study will determine if AVONEX is beneficial in delaying the onset of clinically definite multiple sclerosis in subjects who have experienced a first and recent onset of a demyelinating event, and who are at high risk for multiple sclerosis. A total of 380 subjects will be treated with IM injections of AVONEX or placebo weekly for up to three years.